As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS NOT GROUNDED: AT CRUISE, WE GOT A MASTER WARNING, WITH A LAVATORY SMOKE EICAS FOR 10-20 SECONDS. FLIGHT ATTENDANT PANEL SHOWED RIGHT AFT LAVATORY. ALL LAVATORIES INSPECTED WERE OK. ACCOMPLISHED OPERATIONAL TEST OF THE LAVATORY SMOKE-DETECTION SYSTEM IAW AMM 26-17-00-710-001-A.